As is known, in the field of the processing of plastic materials of the hygroscopic type (i.e., capable of absorbing moisture internally), one very important step is the process for dehumidifying the granules of plastic material before processing them.
It is necessary to eliminate the moisture from the granules because during the melting facilitated by the high temperature the water enters the molecular chain of the polymer, breaking it and thus reducing mechanical performance.
At the same time, bubbles, blowholes and surface defects are generated in the melted matrix.
Various dehumidification processes for granules of plastic materials are known; among them, the most widely used one is based on the drying of granules by means of a stream of dry air, which removes the water therefrom.
The dehumidification system is constituted typically by a dehumidification device, commonly known as dryer, from which a delivery duct extends which is inserted within a hopper for containing the mass of granules of plastic material.
A duct for venting the air injected by the dryer extends from the covering of the hopper. This venting duct is connected to the intake of the dryer, and a filter is arranged along the venting duct.
The hopper is constituted by a container for containing granules of plastic material and is provided, in an upper region, with a covering provided with a frustum-shaped manifold for loading the hopper.
The outlet of the delivery duct is provided on the bottom region of the container.
The dryer is provided with an internal apparatus which dehumidifies the air, heats it and pumps it into the lower conical part of the hopper, striking the material contained therein.
The air flows upward through the entire hopper until it gathers at the top of the hopper in order to be collected by a volute formed by the covering and by the sides of the manifold and is then conveyed into the venting duct.
The air that reenters the dryer is the vented air, because the recycled air has a lower moisture content than the ambient air and therefore it is more convenient to recycle it.
The filter is intended to purify the air that leaves the hopper from the dust which is always contained in the materials being dried, preventing the dust from then ending in the circuits of the dryer and damaging it.
The risk of damage is so high that for safety reasons a filter is also usually installed inside the dryer, so as to minimize the possibility of dirtying and damage.
Therefore, at least two filters are installed in every system: one on the venting duct of the hopper, which returns to the dryer, and one inside the dryer.
The filters arranged downstream of the hopper are constituted generally by a shell which can be opened and by a cartridge made of filtering material; the assembly forms an element which is physically distinct from the hopper, to which it is fixed externally.
Therefore, the filter is an element of the system which is installed externally with respect to the hopper and is connected to the ducts during the assembly of the system.
This operation entails costs due to the purchase of the filter, to assembly, and to an increase in the circuit complexity of the system and in heat loss.
Problems of this type occur also in similar dehumidification systems used for other types of particulate material.